Defibrillator
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A defibrillator is a medical device used in the defibrillation of the heart. It consists of a central unit and a set of two electrodes. The central unit provides a source of power and control. The two electrodes are placed directly on or in the patient. The device is designed to deliver an electric shock to the patient, in an effort to stop ventricular fibrillation or pulseless ventricular tachycardia. It is also used to stop the heart in an effort to allow it to reset its natural pacemaker in supraventricular tachycardia. Ventricular fibrillation is a situation of electrical chaos in the heart's conduction system, in essence distorting the coordinated contraction of cardiac muscular tissue. This leads to a situation where the heart produces minimal or no forward blood flow, causing circulatory arrest and death within minutes from hypoxic brain damage. The defibrillation current depolarizes the entire electrical system of the heart causing a complete cessation of electrical activity. This, in turn, gives the opportunity of impulses from the normal conduction pathways to regain control of the muscular tissue of the heart.
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[edit] Types of Defibrillators
[edit] Internal Defibrillators
The device may be implanted directly in the user of the device. If so, it is known as an implantable cardioverter-defibrillator (ICD). This type of defibrillator is designed to provide immediate defibrillation to high-risk patients. By actively monitoring the pulse rate, rhythm, and waveform, and by comparing atrial and ventricular activity, an ICD can detect ventricular fibrillation, and immediately initiate defibrillation[1][2].
[edit] External Defibrillators
External defibrillators are typically used in hospitals or ambulances, but are increasingly common outside the medical realm, as automated external defibrillators become safer and cheaper. There are a variety of technologies and form factors in use for external defibrillators, and recent progress in cardiac research has led to substantial improvements in the underlying technology.
[edit] Biphasic Defibrillation
Until recently, external defibrillators relied on monophasic shock waves. Electrical pulses are sent rapidly from one electrode to the other, only in one direction. Biphasic defibrillation, however, alternates the direction of the pulses, completing one cycle in approximately 10 milliseconds. Biphasic defibrillation was originally developed and used for implantable cardioverter-defibrillators. When applied to external defibrillators, biphasic defibrillation significantly decreases the energy level necessary for successful defibrillation. This, in turn, decreases risk of burns and myocardial damage.
Ventricular fibrillation (VF) could persist in about 40% of cardiac arrest patients treated with a single shock from a monophasic defibrillator. Most biphasic defibrillators have a first shock success rate (VF is eliminated and a normal rhythm can be returned) of greater than 90%.[3]
[edit] Automated External Defibrillators
An Automated External Defibrillator (AED) is a self-contained defibrillator device designed for portability and ease of use. AEDs are generally shaped like a briefcase, so that they may be carried easily by a handle. An AED contains a battery, a control computer, and electrodes. Upon placing the electrodes on the patient, the control computer in an advanced system will assess the patient, determining the type of rhythm or arrhythmia present. It will then set appropriate power levels and signal that a shock is needed. If the patient does not require defibrillation, units will not allow a shock to be administered. A button must still be depressed manually to trigger the shock, as the operator beforehand must be certain that no-one is touching the patient at that time. Often, AEDs will have speakers which give audible commands when the AED is opened.
Current AED devices are designed for emergency medical technicians, home users, public safety officers and other people with minimal medical knowledge. AEDs are available for $1000 for a basic model to several thousand dollars for a more fully-featured or durable model.
[edit] Electrodes
The electrode is a key part of any defibrillation system. The proper selection and placement of electrodes can determine the effectiveness of the procedure.
[edit] Design
The most well-known type of electrode is the traditional metal paddle with an insulated handle. This type must be held in place on the patient's skin while a shock or a series of shocks is delivered. Before the paddle is used, a gel must be applied to the patient's skin, in order to ensure a good connection and to minimize electrical resistance, also called chest impedance (despite the DC discharge).
Another type of resuscitation electrode is designed as an adhesive pad. When a patient has been admitted due to heart problems, and the physician or nurse has determined that he or she is at risk of arrhythmia, they may apply adhesive electrodes to the patient in anticipation of any problems that may arise. These electrodes are left connected to a defibrillator. If defibrillation is required, the machine is charged, and the shock is delivered, without any need to apply any gel or to retrieve and place any paddles.
Both solid- and wet-gel adhesive electrodes are available. Solid-gel electrodes are more convenient, because there is no need to clean the patient's skin after removing the electrodes. However, the use of solid-gel electrodes presents a higher risk of burns during defibrillation, since wet-gel electrodes more evenly conduct electricity into the body.
Adhesive electrodes are designed to be used not only for defibrillation, but also for non-invasive pacing and electrical cardioversion.
While the paddles on a Monitor/Defibrillator may be quicker than using the patches, adhesive patches are superior due to their ability to provide appropriate EKG tracing without the artifact visible from human interference with the paddles. Many monitor defibrillators provide three, five or 12-lead EKG monitoring to compensate for this downfall of the paddles. Adhesive electrodes are also inherently safer than the paddles for the operator of the defibrillator to use, as they minimize the risk of the operator coming into physical (and thus electrical) contact with the patient as the shock is delivered, by allowing the operator to stand several feet away. Another inconvenience of the paddles is the requirement of around 25lbs of pressure to be applied while defibrillating.
[edit] Placement
Resuscitation electrodes are placed according to one of two schemes. The anterior-posterior scheme (conf. image) is the preferred scheme for long-term electrode placement. One electrode is placed over the left precordium (the lower part of the chest, in front of the heart). The other electrode is placed on the back, behind the heart in the region between the scapula. This placement is preferred because it is best for non-invasive pacing.
The anterior-apex scheme can be used when the anterior-posterior scheme is inconvenient or unnecessary. In this scheme, the anterior electrode is placed on the right, below the clavicle. The apex electrode is applied to the left side of the patient, just below and to the left of the pectoral muscle. This scheme works well for defibrillation and cardioversion, as well as for monitoring an ECG.
[edit] Inventor
The first case of a human life saved by defibrillation was reported by Beck et al in 1947. Dr. Claude Beck successfully revived a patient in an operating room using an open-chest electric defibrillation device, which had been designed by him and then built by his friend James Rand.
The first man to discover that DC electricity is most effective for treating arrhythmias like ventricular fibrillation and ventricular tachycardia is Bernard Lown. AC electricity cannot be used because it is considerably more likely to induce ventricular fibrillation rather than terminating it.
The closed-chest defibrillator device was pioneered by Dr V. Eskin with assistance by A. Klimov in the city of Frunze, USSR in mid 1950s. Successors of this device continue to be used to this day.
External defibrillators were developed for NASA to be used in their human centrifuge at Moffat Field in Mountain View, California,, by Tom Corbin in 1956. These external units were also used by W.B.Kouwenhoven at Johns Hopkins University in his development of the technique now known as CPR.
Another advance was the development of a mobile defibrillator in 1966. The device was suitable for installation in ambulance vehicles and was developed by cardiologist Frank Pantridge in Belfast, Northern Ireland. This is one of the innovations that led to modern EMS and, in the late 1990s, the mobilization of advanced cardiac life support with paramedics.
[edit] Defibrillators in popular culture
The defibrillator was first seen on film in the 1966 movie Fantastic Voyage. Since then they have appeared in many modern television and film medical programs. Its use in the near-death experience experiments by the characters of the 1990 movie Flatliners made the defibrillator virtually one of the co-stars of that film. It was also utilized in plotlines in several films and TV series.
In the television series Emergency!, firefighters John Gage and Roy DeSoto often used defibrillators, starting the film tradition of yelling "Clear" right before applying the shock, to warn everyone around to stay away from the patient for risk of electrical shock.
In the same series, the defibrillator induces a sudden, violent jerk or convulsion by the patient; in reality, although the muscles may contract, such dramatic patient presentation is rare.
Most television shows will have the medical provider defibrillate the "flat-line" ECG rhythm (also known as asystole); this is not done in real life. Only the cardiac arrest rhythms ventricular fibrillation and pulseless ventricular tachycardia are normally defibrillated. (There are also several heart rhythms that can be "defibrillated" when the patient is not in cardiac arrest, such as supraventricular tachycardia or ventricular tachycardia that produces a pulse, though the procedure is then known as cardioversion.) However, a flatline may actually be a fibrillation that is too weak to be seen on the monitor (fine v-fib), so a shock may be delivered, but it is not regarded as the treatment of choice, as the probability of a successful conversion is very small. According to the current guidelines, in this situation, continued CPR in order to improve the oxygenation of the heart for a few minutes is preferred before defibrillation is attempted.
During the penultimate season of M*A*S*H, in the episode "Heroes", B. J. Hunnicutt saves a patient's life by using a hastily-assembled defibrillator after reading an article in a medical journal (contrary to history, Hawkeye Pierce suggests that the technique had previously only been tested on dogs). His colleague claims he's just invented a new way of saving lives.
In the 1988 film Short Circuit 2, Benjamin used a defibrillator to provide the robot Johnny 5 with temporary power when its leaking battery ran out of power. It also played a chief role in a scene in the 1989 film The Abyss, when Lindsey Brigman allowed herself to be drowned in freezing water so she and her husband, Bud, could both make it back to the Deep Core underwater drilling station.
In the computer games Battlefield 2 and Battlefield 2142, the defibrillator is a tool available for medics. Contrary to reality, they are surprisingly convenient and mobile tools that can quickly revive a fallen comrade, or kill an enemy soldier with a single jolt.
In the 2006 James Bond movie Casino Royale, Bond uses a defibrillator from his Aston Martin DBS, to revive himself after being poisoned with digitalis during his poker game. On returning to the table he wittily remarks "Sorry; that last hand -- nearly killed me."
[edit] See also
- cardiopulmonary resuscitation (CPR)
- Advanced cardiac life support (ACLS)
- Cardioversion
- Implantable cardioverter-defibrillator
- Defibrillation
- Automated external defibrillator
- Heart Attack
- Packer Whacker
[edit] References
Advanced Cardiac Life Support- American Heart Association
[edit] External links
- How an internal defibrillator is implanted from Children's Hospital Heart Center, Seattle.
